The present disclosure relates to the use of certain solvents in dampening fluids used during variable data lithographic printing. This disclosure also relates to apparatuses using such dampening fluids, and methods of using such dampening fluids, such as in variable lithographic printing applications.
Offset lithography is a common method of printing today. (For the purposes hereof, the terms “printing” and “marking” are interchangeable.) In a typical lithographic process a printing plate, which may be a flat plate, the surface of a cylinder, or belt, etc., is formed to have “image regions” formed of a hydrophobic/oleophilic material, and “non-image regions” formed of a hydrophilic/oleophobic material. The image regions correspond to the areas on the final print (i.e., the target substrate) that are occupied by a printing or marking material such as ink, whereas the non-image regions correspond to the areas on the final print that are not occupied by said marking material. The hydrophilic regions accept and are readily wetted by a water-based fluid, commonly referred to as a dampening fluid or fountain fluid (typically consisting of water and a small amount of alcohol as well as other additives and/or surfactants to reduce surface tension). The hydrophobic regions repel dampening fluid and accept ink, whereas the dampening fluid formed over the hydrophilic regions forms a fluid “release layer” for rejecting ink. The hydrophilic regions of the printing plate thus correspond to unprinted areas, or “non-image areas”, of the final print.
The ink may be transferred directly to a target substrate, such as paper, or may be applied to an intermediate surface, such as an offset (or blanket) cylinder in an offset printing system. The offset cylinder is covered with a conformable coating or sleeve with a surface that can conform to the texture of the target substrate, which may have surface peak-to-valley depth somewhat greater than the surface peak-to-valley depth of the imaging plate. Also, the surface roughness of the offset blanket cylinder helps to deliver a more uniform layer of printing material to the target substrate free of defects such as mottle. Sufficient pressure is used to transfer the image from the offset cylinder to the target substrate. Pinching the target substrate between the offset cylinder and an impression cylinder provides this pressure.
Typical lithographic and offset printing techniques utilize plates which are permanently patterned, and are therefore useful only when printing a large number of copies of the same image (i.e. long print runs), such as magazines, newspapers, and the like. However, they do not permit creating and printing a new pattern from one page to the next without removing and replacing the print cylinder and/or the imaging plate (i.e., the technique cannot accommodate true high speed variable data printing wherein the image changes from impression to impression, for example, as in the case of digital printing systems). Furthermore, the cost of the permanently patterned imaging plates or cylinders is amortized over the number of copies. The cost per printed copy is therefore higher for shorter print runs of the same image than for longer print runs of the same image, as opposed to prints from digital printing systems.
Accordingly, a lithographic technique, referred to as variable data lithography, has been developed which uses a non-patterned reimageable surface that is initially uniformly coated with a dampening fluid layer. Regions of the dampening fluid are removed by exposure to a focused radiation source (e.g., a laser light source) to form pockets. A temporary pattern in the dampening fluid is thereby formed over the non-patterned reimageable surface. Ink applied thereover is retained in the pockets formed by the removal of the dampening fluid. The inked surface is then brought into contact with a substrate, and the ink transfers from the pockets in the dampening fluid layer to the substrate. The dampening fluid may then be removed, a new uniform layer of dampening fluid applied to the reimageable surface, and the process repeated.
The patterning of dampening fluid on the reimageable surface member in variable data lithography essentially involves using a laser or some other energy source to selectively boil off or ablate the dampening fluid in selected locations. This process can be energy intensive due to the large latent heat of vaporization of water. At the same time, high-speed printing necessitates the use of high-speed modulation of that energy source, which can be prohibitively expensive for high power lasers. Therefore, from both an energy and cost perspective, it would beneficial to reduce the total amount of energy that is needed to achieve pattern-wise vaporization of the dampening fluid.
The essential role of the dampening fluid in both traditional offset printing and in variable lithographic printing is to provide selectivity for the imaging and transfer of the ink. Dampening fluid generally contains water and some additives to reduce surface tension, such as a surfactant. The dampening fluid acts as a low viscosity release layer film which preferentially splits at the inking nip, thus preventing ink adhesion to the imaging member surface. In addition, the dampening fluid is to a large degree immiscible with the ink chemistry, being oleophobic in its chemical nature. Otherwise, the dampening fluid can break apart into small emulsified droplets with the ink which can lead to background tinting.
As already discussed above, an additional consideration in variable lithographic printing is the energy necessary to boil off the dampening fluid. For example, water is a very polar molecule and has both high surface tension and a high latent heat of vaporization, which relates to the energy required to change water from its liquid phase to its vapor phase. The high heat of vaporization leads to high energy requirements for the laser used to vaporize the dampening fluid.
A further consideration related to the thermal properties of the dampening fluid is the dampening fluid boiling temperature. Too low a boiling temperature will mean quick thinning of the fluid due to its partial pressure evaporation near room temperature. It is desirable to have a high enough boiling temperature such that the evaporation rate does not compete with the laser boiling process because this insures better image definition. On the other hand, too high a boiling temperature means added laser energy is necessary due to the specific heat of the dampening fluid required for raising its temperature up to the boiling point, and this can therefore reduce the overall printing speed for a given laser power. The high surface tension of water causes the dampening fluid to tend to bead, rather than to spread evenly over the surface of the imaging member. To reduce the surface tension, dampening fluid usually includes another solvent which is less polar than water, such as isopropanol (IPA). However, isopropanol is a volatile organic compound (VOC), and its emission is regulated. In addition variable lithographic imaging typically uses an elastomeric surface, and IPA is known to cause swelling in may many elastomeric surface materials. Other aqueous-based surfactants tend to have high boiling points and therefore leave a residue behind on the surface of the imaging member, compromising the integrity of the imaging member for making images of suitable quality.
It would be desirable to provide dampening fluids that can avoid such problems for variable lithographic printing.